1. Field of Invention
The present invention relates to a clock generator. More particularly, the present invention relates to a method for adjusting a spread amount of the clock generator.
2. Description of Related Art
In modern electronic devices, the transfer rate of the 10 interface, such as Serial Advanced Technology Attachment (SATA) interface, has been increased up to Gbps level. SATA is mainly used in hard disk data access, its speed has reached 6 Gbps. The maximum transmission speed of the SATAI, SATAII, SATAIII is 1.5 Gbps, 3 Gbps, and 6 Gbps, respectively.
As the SATA transfer rates increases, the Electro-Magnetic Interference (EMI), affecting or destroying the performance of peripheral electronic components, gets even worse, and the peripheral electronic components can not be used as a result. Furthermore, the clock signal generated by the clock generator is the main source causing the EMI phenomenon, so it is important to reduce the EMI phenomenon caused by the clock generator.
Presently, Spread Spectrum Clock Generator (SSCG) is commonly used to reduce the EMI phenomenon. As the name suggests, “Spread spectrum” means to spread the center frequency of the clock signal with less amount, which reduces the energy of the main frequency and disperses the reduced energy to other frequencies than the main frequency. Since the energy of the main frequency has been reduced, so the EMI phenomenon can be reduced as a result.
Spread spectrum causes the variation of the clock signal. Although the spread spectrum reduces the EMI effects, the jitter of the clock signal deteriorates, such that users often need to compromise between the clock jitter and the spread spectrum. However, the amount of the spread spectrum of the clock generator is fixed, the user can not adjust the amount of spread spectrum in accordance with the actual situation. Therefore, users can not trade-off between the amount of the spread spectrum and the clock jitter, which reduces the overall performance.